Signal generating mechanism

ABSTRACT

A signal generating mechanism is disclosed for producing an electrical signal having a frequency proportional to the angular velocity of a rotating shaft. The signal generating mechanism may be used in the distributor for a breakerless ignition system for a multi-cylinder internal combustion engine, in which case the frequency of the alternating signal is equal to the rate at which ignition sparks are to be generated. The signal generating mechanism includes a lower plate and bushing through which the rotating shaft passes. A non-adjustable stator assembly is formed by a hub positioned around the bushing, the hub having an upper plate attached to it on which is affixed a permanent magnet. A stator is positioned on the permanent magnet and has an extending pole-piece that passes through an electrical pickup coil. An armature, or rotor, is formed by a toothed-wheel that is affixed to the rotating shaft. The teeth of the toothed-wheel come into and go out of alignment with the portion of the stator pole-piece that extends through the electrical coil.

United States Patent [191 Kostan Jan. 1, 1974 1 SIGNAL GENERATINGMECHANISM Charles C. Kostan, Detroit, Mich.

[73] Assignee: The Ford Motor Company,

Dearborn, Mich.

[22] Filed: Dec. 20, 1972 [21] Appl. No.: 316,945

[75] Inventor:

[52] US. Cl. 310/70, 123/148 [51] Int. Cl. H02k 11/00 [58] Field ofSearch 310/68 R, 68 C, 69,

[5 6] References Cited UNITED STATES PATENTS 8/1964 Race 310/70 R 2/1966Tyzack 310/70 R 5/1969 Falge 310/70 R Primary Examiner-J. D. MillerAssistant Examiner-Robert J. Hickey Attorney-Keith L. Zerschling et a1.

[ 5 7 ABSTRACT A signal generating mechanism is disclosed for producingan electrical signal having a frequency proportional to the angularvelocity of :a rotating shaft. The signal generating mechanism may beused in the distributor for a breakerless ignition system for amulticylinder internal combustion engine, in which case the frequency ofthe alternating signal is equal to the rate at which ignition sparks areto be generated. The signal generating mechanism includes a lower plateand bushing through which the rotating shaft passes. A non-adjustablestator assembly is formed by a hub positioned around the bushing, thehub having an upper plate attached to it on which is affixed a permanentmagnet. A stator is positioned on the permanent magnet and has anextending pole-piece that passes through an electrical pickup coil. Anarmature, or rotor, is formed by a toothed-wheel that is affixed to therotating shaft. The teeth of the toothed-wheel come into and go out ofalignment with the portion of the stator pole-piece that extends throughthe electrical coil.

5 Claims, 7 Drawing Figures PATENTED JAN 1 I974 SHEET 10F 4 PATENTEUJAHnan sum 30; 4 3'783'314 PAIENTEDJAK H974 3.783.314

SHEET HIP 4 1 SIGNAL GENERATING MECHANISM BACKGROUND OF THE INVENTIONThis invention relates to a signal generating mechanism for producing analternating electrical signal having a frequency proportional to theangular velocity of a rotating shaft. More particularly, the inventionrelates to a signal generating mechanism particularly suitable for usein a distributor for a breakerless ignition system for a multi-cylinderinternal combustion engine.

Common past and present practice in ignition systems for multi-cylinderinternal combustion engines has been to employ a set of breaker pointsin a distributor to generate sparks as required by the engine.Electronic ignition systems eliminating these breaker points recentlyhave been developed. These electronic breakerless ignition systemsrequire a signal generating mechanism capable of indicating the times orinstants at which it is desired to generate sparks in the various enginecombustion chambers. A common feature of such signal generating devicesis that they produce an electrical signal having a frequencyproportional to the.

angular velocity of a rotating shaft, the shaft usually being the shaftin a distributor which rotates at an angular velocity equal to that ofthe engines cam shaft.

Although various signal generating mechanisms have been proposed for usewith multi-cylinder internal combustion engines, the present inventionis directed to the type which employs a toothed-wheel affixed to therotating shaft. The toothed-wheel forms the annature, or rotor, of analternating signal generator. The stator assembly of the alternatingsignal generator includes an electrical pickup coil through which astator pole-piece passes. The teeth of the toothed-wheel come into andgo out of alignment with the stator pole-piece to vary the flux producedby a permanent magnet having a magnetic circuit including the stator andits polepiece and the rotating toothed-wheel. This produces analternating electrical signal across the terminals of the electricalcoil. This alternating signal, in turn, may be coupled to a breakerlessignition system capable of producing sparks in relation to thealternating signal, for example, at one or more selected points in thesignal waveform.

SUMMARY OF THE INVENTION In accordance with the invention, a signalgenerating mechanism for producing an alternating electrical signalhaving a frequency proportional to the angular velocity of a rotatingshaft comprises a lower plate that is fixed relative to the rotatingshaft. The lower plate has an annular opening therein and a bushing isaffixed in the annular opening. The shaft passes through the bushing. Astator assembly is positioned above the lower plate. The stator assemblyincludes a hub which is positioned around a portion of the bushingaffixed to the lower plate. Attached to the hub is an upper plate,parallel with the lower plate, on which is positioned a permanent magnethaving a magnetic field that is perpendicular to the upper plate. Thestator assembly fur ther includes the stator itself, which is positionedin contact with the permanent magnet, the permanent magnet being locatedintermediate the stator and the upper plate. The stator has a pole-piecewhich passes through and supports an electrical coil the axis of whichis perpendicular to the shaft axis. An armature, or rotor, is formed bya toothed-wheel that is affixed to the shaft for rotation therewith. Thetoothed-wheel is concentric with the shaft axis and the teeth of thetoothed-wheel are positioned to come into and to go out of alignmentwith the pole-piece of the stator.

Rotation of the shaft and the toothed-wheel attached to it produces avoltage across the terminals of the electrical coil. This voltage isdetermined in accordance with the well known equation E= -N dda/dt whereE is the electromotive force produced across the terminals of theelectrical coil, N is the number of turns in the electrical coil anddldt is the time rate of change of magnetic flux.

The entire stator assembly is non-adjustably secured together, thus,eliminating screw adjustments or the like. Prior art devices haverequired adjustments of this type to obtain voltage signals of a desiredlevel. The elimination of such adjustments simplifies the man ufactureof the signal generating mechanism and eliminates the possibility ofmaladjustment during use of the device The invention will be betterunderstood by reference to the detailed description which follows and tothe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view, thesection being taken along the line 1-1 of FIG. 2, of an ignitiondistributor for a multi-cylinder internal combustion engine, thedistributor including a signal generating mechanism for producing analternating electrical signal having a frequency proportional to theangular velocity of the rotatable distributor shaft;

FIG. 2 is a plan view of the distributor of FIG. 1;

FIG. 3 is a sectional view of the distributor, the section being takenalong the line 3-3 in FIG. 1, and illustrates a centrifugal weightmechanism for timing advance;

FIG. 4 is an enlarged plan view of the stator assembly used in thesignal generating mechanism of the distributor of FIG. 1; 7

FIG. 5 is an enlarged elevational view, partially in section, of thestator assembly of FIG. 4;

FIG. 6 is an enlarged pictorial view of the stator assembly of FIGS. 4and 5; and

FIG. 7 is an illustration of the alternating signal waveform that may beproduced by the signal generating mechanism in the ignition systemdistributor.

DETAILED DESCRIPTION With particular reference now to FIGS. 1 through 3,there is shown an ignition system distributor 10 for supplying sparks toa multi-cylinder internal combustion engine. The distributor 10 includesa housing 12 having a cylindrical bearing 14 positioned therein. A shaft16 is rotatably joumalled within the bearing 14. The shaft 16 is drivenby a gear 20 that, in use, meshes with another gear (not shown), drivenby the internal combustion engine. The shaft 16 has a reduced-diameterportion 18, and both the larger-diameter and reduced diameter portionsof the shaft contain grooves for lubrication purposes.

A ferromagnetic sleeve 22 fits over the reduceddiameter portion 18 ofthe shaft 16. The sleeve 22 is retained 0n the shaft 16 with a wireretainer 24. The sleeve 22 is rotatably mounted on the reduceddiameterportion 18 of the shaft 16, and this rotation relative to the shaft iscontrolled by a centrifugal advance mechanism of the usual design.

The centrifugal advance mechanism, generally designated by the numeral26, comprises a plate 28 affixed to the shaft 16 and a plate 30 affixedto the sleeve 22. In the usual manner, the plates 28 and 30 are coupledtogether by means of springs 32 and 34. In order for the plate 30 andsleeve 22 to rotate about the plate 26 and shaft 16, the force of thesprings must be overcome. When the shaft 16 rotates, a pair of weights36 and 38, pivotally connected to the plate 28, exert a force that actsagainst that of the springs 32 and 34 and tends to rotate the plate 30and sleeve 22 with respect to the shaft 16. The magnitude of this forceis proportional to the shaft angular velocity. This provides acentrifugal advance in the ignition timing. For the purpose of thepresent invention, the sleeve 22 may be regarded as a part of the shaft16 with which it rotates.

The ignition system distributor is shown without the usual cap andhigh-voltage distribution rotor. It should be understood that theseelements would be present in a complete distributor installation. Thedistributor cap would be of the usual configuration in which a pluralityof electrical contacts would be connected by high-voltage leads to sparkplugs for the multi-cylinder internal combustion engine. The highvoltagedistribution rotor would be secured to the sleeve 22 and would rotatewith it to distribute voltage from the high-voltage side of an ignitioncoil to the electrical leads to the various spark plugs.

The ignition system distributor 10 includes a signal generatingmechanism of the invention, generally designated by the numeral 40, forproducing an alternating electrical signal having a frequencyproportional to the angular velocity of the rotating shaft 16 and sleeve22. Preferably, the alternating signal generating mechanism has afrequency equal to the rate at which sparks are to be generated by theignition system. Typically, this frequency is equal to the speed of theshaft in revolutions per second times the number of cylinders in theinternal combustion engine.

The signal generating mechanism 40 illustrated in the drawings isintended for an S-cylinder engine. It comprises a toothed-wheel 42,hereinafter referred to as a rotor, and a stator assembly 44. The rotor42 is attached to the sleeve 22 for rotation therewith by means of aroll pin 46. Also, the position of the rotor 42 on the axis of thesleeve 22 is determined by a rotor ring retainer 48. The rotor 42 may bemade from a sintered high-permeability material and has eight radiallyextending teeth 50, these corresponding in number to the number ofcylinders in the engine. Preferably, to achieve a desirable signal, theteeth extend radially outward a distance of from about five to times thedimension of the air-gap 51 between the rotor and stator. Also, it ispreferred that the air-gap 51 have a dimension ranging from a maximum of0.062 in. to a minimum of 0.023 in., the midpoint of this range beingmost satisfactory. In this connection, it should be noted that similarsignal generating mechanisms of the prior art were limited to air-gapdimensions ranging from about 0.006 in. to 0.001 in. and requiredadjustment means to maintain this dimension.

The stator assembly 44 is located and positioned by means of a lowerplate 52 that has an annular opening therein in which an annular bushing54, made from a ferromagnetic material, is fixed. The shaft 16 and associated sleeve 22 pass through the bushing 54 and rotate freely withinit. The lower plate 52 is positioned perpendicularly to the axis of theshaft and is secured to the distributor housing 12 by a plurality ofscrews 56. The bushing 54 has a portion which extends above the lowerplate 52. This upwardly extending portion of bushing 54 has lubricationgrooves 53 on its radially outward exterior that are filled with greaseduring the assembly of the mechanism.

The stator assembly 44 may best be seen in the enlarged views of FIGS. 4through 6. It includes an annular hub 58, also made from a ferromagneticmaterial, that is positioned around the bushing 54 so that the statorassembly may rotate about the bushing. A wire ring retainer 60 (FIG. 1)is used to hold the stator assembly 44 in position on the bushing 54.The stator assembly 44 comprises the hub 58, an irregularly shaped,ferromagnetic upper plate 62 positioned parallel to the lower plate 52,a permanent magnet 64, a stator 66 and an electrical pickup coil 68.

The upper plate 62 of the stator assembly is securely staked inlocations 70 to the hub 58. A pin 72 extends upwardly from the upperplate 62. In FIG. 2, it may be seen that a rod 74 of a vacuum motor 76,used in addition to the centrifugal timing mechanism 26 forigniti0n-timing advance and retard, is pivotally connected to the pin 72on the upper plate 62. The rod 74 controls rotation of the hub 58, andthe remainder of the stator assembly 44, about the bushing 54.

The permanent magnet 64 has an arcuate shape. Preferably, it is madefrom a ferrite material, such as sintered ceramic strontium ferrite. Itis preferred that the permanent magnet 64 be magnetized at assembly ofthe signal generating mechanism. Its magnetic field should be orientedsuch that its direction through the magnet is perpendicular to the upperplate 62. Accordingly, the upper surface 78 of the permanent magnet 64may be made a south pole and the lower surface, that is, the surface incontact with the upper plate 62, may be made a north pole. In such case,an imaginary line interconnecting these poles is perpendicular to theupper plate 62. The permanent magnet 64 preferably is magnetized to aninduction of about 3600 gauss.

The stator 66 and the permanent magnet 64 are nonadjustably secured tothe upper plate 62 of the stator assembly with rivets 80 and 82. Toavoid impact damage to the permanent magnet, a spin-type rivetingprocess may be used. The stator 66 is made from a ferrogmagneticmaterial and preferably is made from a sintered-iron, highpermeability,low-carbon material hav ing a minimum density of about 6.8 grams percubic centimeter. The stator 66 has a generally arcuate shape but alsohas a pole-piece 86 of rectangular crosssection extending through thecentral region of the electrical coil 68 and radially toward the axis ofthe shaft 16. The teeth 50 of the rotor 42 come into and go out ofalignment with the pole-piece 86 of stator 66 as the shaft 16 and itssleeve 22 rotate. It should be noted that the axis of rotation of thestator assembly 44 about the bushing 54 is the same as the axis of shaft16, and, thus, the air gap between the rotor teeth and pole-piece doesnot change during ignition timing changes.

The electrical pickup coil 68 is supported by and surrounds the statorpole-piece 86 and preferably is formed from a helical winding of fromabout 4,000 to 4,075 turns of gauge number 39 (American Wire Gauge)copper wire and may have a resistance of from 600 to 700 ohms.Preferably, the electrical pickup coil 68 is encapsulated by molding ina suitable insulating material. The pickup coil 68 has leads 9t) and 92connected to the electrical winding terminals that extend out of thecoil encapsulation material. Also, the pickup coil has a hook 94attached to it. A spring clip 96 is connected to the hook 94 and to thestator pole-piece 86 to hold the electrical pickup coil on thepole-piece 86 in a secure manner. The axis of the coil 68 isperpendicular to the axis of the shaft 16..

In FIG. 2, it may be seen that the wires 90 and 92 enter and passthrough a grommet 98, preferably made from a molded polyvinyl chloridematerial, to a connector 100. The grommet 98 contains a third wire 102that is connected by one of the screws 56 to the lower plate 52 toestablish an electrical ground. The Wire 102 extends out of the grommet98 and into the connector 100. The wire 102 is a ground lead.

In operation, the rotor 42 preferably turns in the counterclockwisedirection indicated in FIG. 2 by the arrow on one of its teeth. As therotor 42 rotates, the magnetic flux changes to produce a time varyingvoltage across the electrical leads 90 and 92 of the electrical coil 68.This voltage or EMF is produced in accordance with the aforementionedequation E= -N dqb/dt.

The magnetic circuit is illustrated by the dotted line 104 in FIG. 6.This magnetic circuit includes the stator 66 and its pole-piece 86, thetoothed rotor 42 and shaft 16 (not shown in FIG. 6), and the upper plate62 that is in contact with the bottom surface of the permanent magnet64.

With reference in particular to FIGS. 2 and 7, wherein one of the teeth50 of the rotor 42 is in alignment with the pole-piece 86 of the stator66, let it be assumed that the rotor is turning in a counterclockwisedirection. At this instant with the tooth in alignment with thepole-piece 86, there is no change in flux with respect to time and thegenerated voltage is zero. This corresponds to the time t 0 of FIG. '7.As the tooth moves out of alignment with the pole-piece 86 of theelectrical coil, a voltage is generated across the coil leads 90 and 92due to the change in magnetic flux. This flux change, and consequentlythe generated voltage, reaches a maximum and then decreases to zero at atime t= T/2 corresponding to the time when the polepiece 86 is midwaybetween two of the teeth 50 of the rotor 42. Continued rotation of therotor 42 produces a change in magnetic flux that generates a voltage ofopposite polarity. This voltage increases until it once again becomeszero at time t= T when the next tooth 50 is in alignment with thepole-piece 86. Thus, an alternating signal is generated having afrequency proportional to the angular velocity of the rotating shaft ofthe distributor.

Based upon the foregoing description of the invention, what is claimedis:

l. A signal generating mechanism for producing an alternating electricalsignal having a frequency proportional to the angular velocity of arotating shaft, said signal generating mechanism comprising, incombination: a lower plate fixed relative to said rotating shaft, saidlower plate having an opening therein; an annular bushing affixed insaid lower plate opening, said bushing extending above said lower plate,said shaft passing through said bushing; a stator assembly, said statorassembly including an annular hub positioned around the portion of saidbushing extending above said lower plate, said hub being rotatable aboutsaid bushing, an upper plate parallel to said lower plate and affixed tosaid hub, a permanent magnet attached to said upper plate, said magnetbeing polarized to produce a mag netic field having a direction throughsaid magnet that is perpendicular to said upper plate, a statorpositioned on said permanent magnet, said stator having a polepiece, andan electrical coil positioned on said stator pole-piece; and a rotorattached to said shaft for rotation therewith, said rotor having atleast one tooth positioned to come into and go out of alignment withsaid stator pole-piece; whereby rotation of said rotor produces analternating electrical signal across the terminals of said electricalcoil having a frequency proportional to the angular velocity of saidrotating shaft.

2. A signal generating mechanism for producing an alternating electricalsignal having a frequency proportional to the angular velocity of arotating shaft, said signal generating mechanism comprising, in combination: a lower plate fixed relative to said rotating shaft, said lowerplate having an opening therein; an annular ferromagnetic bushingaffixed in said lower plate opening, said bushing extending above saidlower plate, said shaft passing through said bushing; a stator assembly,said stator assembly including an annular ferromagnetic hub positionedaround the portion of said bushing extending above said lower plate,said hub being rotatable about said bushing, a ferromagnetic upper plateparallel to said lower plate and affixed to said hub, a permanent magnetpolarized to produce a magnetic field having a direction through saidmagnet that is perpendicular to said upper plate, a ferromagnetic statorpositioned on said permanent magnet, said stator and said permanentmagnet being non-adjustably secured to said upper plate, said statorhaving a pole-piece extending radially towards said shaft axis, and anelectrical coil positioned on said pole-piece, said pole-piece extendingthrough said electrical coil; and a ferromag netic rotor affixed to saidshaft for rotation therewith, said rotor having a plurality of teethpositioned to come into and to go out of alignment with said pole-pieceof said stator during rotation of said shaft; whereby rotation of saidshaft produces an alternating signal across the terminals of saidelectrical coil.

3. The signal generating mechanism of claim 2 wherein a gap existsbetween a tooth of said rotor that is in alignment with said pole-piece,said gap being within the dimensional range from 0.062 of an inch to0.023 of an inch.

4. A signal generating mechanism for producing an alternating electricalsignal, which comprises, in combination: a housing; a shaft rotatablyjournalled in said housing; a lower plate secured to said housing, saidlower plate having an opening therein and being positioned perpendicularto said shaft; a ferromagnetic bushing affixed in said lower plateopening, said bushing extending above said lower plate and havinglubrication grooves on the radially exterior portion thereof, said shaftpassing through said bushing; a ferromagnetic sleeve on the portion ofsaid shaft passing through said bushing, said sleeve being rotatableabout said shaft; a centrifugal mechanism interconnecting said sleeveand said shaft for controlling rotation of said sleeve about said shaft;a toothed ferromagnetic rotor secured to said sleeve for rotationtherewith, the teeth of said rotor extending radially outward from theaxis of said shaft; a stator assembly, said stator assembly ineluding anannular ferromagnetic hub positioned around the portion of said bushingextending above said lower plate, said hub being rotatable about saidbushing, a ferromagnetic upper plate parallel to said lower plate andaffixed to said hub for rotation therewith, an arcuately-shapedpermanent magnet positioned on said upper plate, said permanent magnethaving a north pole and a south pole, an imaginary line interconnectingsaid north and south poles being perpendicular to said upper plate, saidpermanent magnet being made from a ceramic ferrite material, aferromagnetic stator positioned on said permanent magnet, said statorand permanent magnet being non-adjustably secured to said upper plate,said stator having a polepiece positioned to permit said rotor teeth tocome into and to go out of alignment with it, and an electrical coilsurrounding said pole-piece of said stator, said electrical coil beinghelically wound and having an axis perpendicular to the axis of saidshaft; and means connected to said upper plate of said stator assemblyfor controlling the rotation of said stator assembly about said bushing.

5. The signal generating mechanism of claim 4 wherein a gap existsbetween a tooth of said rotor that is in alignment with said pole-piece,said gap being within the dimensional range from 0.062 of an inch to0.023 of an inch.

1. A signal generating mechanism for producing an alternating electricalsignal having a frequency proportional to the angular velocity of arotating shaft, said signal generating mechanism comprising, incombination: a lower plate fixed relative to said rotating shaft, saidlower plate having an opening therein; an annular bushing affixed insaid lower plate opening, said bushing extending above said lower plate,said shaft passing through said bushing; a stator assembly, said statorassembly including an annular hub positioned around the portion of saidbushing extending above said lower plate, said hub being rotatable aboutsaid bushing, an upper plate parallel to said lower plate and affixed tosaid hub, a permanent magnet attached to said upper plate, said magnetbeing polarized to produce a magnetic field having a direction throughsaid magnet that is perpendicular to said upper plate, a statorpositioned on said permanent magnet, said stator having a pole-piece,and an electrical coil positioned on said stator pole-piece; and a rotorattached to said shaft for rotation therewith, said rotor having atleast one tooth positioned to come into and go out of alignment withsaid stator pole-piece; whereby rotation of said rotor produces analternating electrical signal across the terminals of said electricalcoil having a frequency proportional to the angular velocity of saidrotating shaft.
 2. A signal generating mechanism for producing analternating electrical signal having a frequency proportional to theangular velocity of a rotating shaft, said signal generating mechanismcomprising, in combination: a lower plate fixed relative to saidrotatiNg shaft, said lower plate having an opening therein; an annularferromagnetic bushing affixed in said lower plate opening, said bushingextending above said lower plate, said shaft passing through saidbushing; a stator assembly, said stator assembly including an annularferromagnetic hub positioned around the portion of said bushingextending above said lower plate, said hub being rotatable about saidbushing, a ferromagnetic upper plate parallel to said lower plate andaffixed to said hub, a permanent magnet polarized to produce a magneticfield having a direction through said magnet that is perpendicular tosaid upper plate, a ferromagnetic stator positioned on said permanentmagnet, said stator and said permanent magnet being non-adjustablysecured to said upper plate, said stator having a pole-piece extendingradially towards said shaft axis, and an electrical coil positioned onsaid pole-piece, said pole-piece extending through said electrical coil;and a ferromagnetic rotor affixed to said shaft for rotation therewith,said rotor having a plurality of teeth positioned to come into and to goout of alignment with said pole-piece of said stator during rotation ofsaid shaft; whereby rotation of said shaft produces an alternatingsignal across the terminals of said electrical coil.
 3. The signalgenerating mechanism of claim 2 wherein a gap exists between a tooth ofsaid rotor that is in alignment with said pole-piece, said gap beingwithin the dimensional range from 0.062 of an inch to 0.023 of an inch.4. A signal generating mechanism for producing an alternating electricalsignal, which comprises, in combination: a housing; a shaft rotatablyjournalled in said housing; a lower plate secured to said housing, saidlower plate having an opening therein and being positioned perpendicularto said shaft; a ferromagnetic bushing affixed in said lower plateopening, said bushing extending above said lower plate and havinglubrication grooves on the radially exterior portion thereof, said shaftpassing through said bushing; a ferromagnetic sleeve on the portion ofsaid shaft passing through said bushing, said sleeve being rotatableabout said shaft; a centrifugal mechanism interconnecting said sleeveand said shaft for controlling rotation of said sleeve about said shaft;a toothed ferromagnetic rotor secured to said sleeve for rotationtherewith, the teeth of said rotor extending radially outward from theaxis of said shaft; a stator assembly, said stator assembly including anannular ferromagnetic hub positioned around the portion of said bushingextending above said lower plate, said hub being rotatable about saidbushing, a ferromagnetic upper plate parallel to said lower plate andaffixed to said hub for rotation therewith, an arcuately-shapedpermanent magnet positioned on said upper plate, said permanent magnethaving a north pole and a south pole, an imaginary line interconnectingsaid north and south poles being perpendicular to said upper plate, saidpermanent magnet being made from a ceramic ferrite material, aferromagnetic stator positioned on said permanent magnet, said statorand permanent magnet being non-adjustably secured to said upper plate,said stator having a pole-piece positioned to permit said rotor teeth tocome into and to go out of alignment with it, and an electrical coilsurrounding said pole-piece of said stator, said electrical coil beinghelically wound and having an axis perpendicular to the axis of saidshaft; and means connected to said upper plate of said stator assemblyfor controlling the rotation of said stator assembly about said bushing.5. The signal generating mechanism of claim 4 wherein a gap existsbetween a tooth of said rotor that is in alignment with said pole-piece,said gap being within the dimensional range from 0.062 of an inch to0.023 of an inch.